could easily be poop pyramid in the black tank :)
Clean blank tank thoroughly. We had the 'smell' only once in over 20 yrs of rv'ing and we did it all....check vents, seals, clean black tank like crazy but still there and then hubby 'cleaned!!' the black tank to the max....amazing what did come out of it--thinking all the time that tank never had that in there. It can easily be in there and if that smell holds then you know something might be down there lurking :)
best of luck on this cause there is nothing worse than that odor when ya hit the bathroom and just want a nice smelling rv and ya get that :)
Once we went heck bent on dealing with the black tank we have been odorless since. Who knows why it did what it did just then, but we got it handled.